From zero to
working software.

export function isAccessible(
  targetId: number,
  fromId: number,
  allLines: ProofLine[],
  closedSubproofs: ClosedSubproof[],
): boolean {
  if (targetId >= fromId) return false

  const targetLine = allLines.find(l => l.id === targetId)
  if (!targetLine) return false

  // A closed subproof's interior is inaccessible to lines OUTSIDE it.
  // Lines inside the same subproof can still reference each other freely.
  for (const sp of closedSubproofs) {
    if (targetId >= sp.startId && targetId <= sp.endId) {
      // Target is inside this closed subproof.
      // Only inaccessible if fromId is OUTSIDE the same subproof.
      const fromIsInside = fromId >= sp.startId && fromId <= sp.endId
      if (!fromIsInside) return false
    }
  }
  return true
}